What happens now is that after you have been logged in past the time limit, you are automatically logged out.

I am not sure what the time limit is, but when you are automatically logged out while you are in the middle of writing a comment, after you have finished and hit the submit button to post it, you will be asked to log in again, and, when you do, your comment will be posted.

It does not evaporate into thin air.

But, just to be safe, if my comment is long, I always copy it before I hit the submit button just in case it might evaporate into thin air.